Influenza activity remains elevated across the country. At this point, the season is considered a “low severity,” but the impact is still significant. For the week, 21.6% of specimens sent to clinical laboratories tested positive for influenza. This is an increase from the percentage reported during the previous week.

For perspective, Alicia noted that we have seen 24–27% positives during the last three seasons. Influenza A (H1) remains the predominant strain reported from public health labs and in 9 of the 10 surveillance regions. Influenza H3 was the predominant strain reported in Region 4, and an increasing percentage of H3 reports are being received from all regions. The amount of reported influenza B circulating viruses continues to remain low, at <3% for the season as a whole. Of these, 57% of the viruses tested at public health labs have been of the Yamagata lineage.

During this season, almost 769 viruses have received antigenic and genetic characterization. This data indicates the majority of viruses are similar to the reference viruses for this season’s vaccine components. All H1 viruses tested have belonged to a single clade, the same clade found in the vaccine virus. Multiple clades are reported from the H3 viruses tested this season. Seventy percent (70%) of the H3 viruses tested were similar to the cell-grown vaccine reference virus. However, only 19% of the H3 viruses were affected by neuraminidase inhibitors against the egg-propogated vaccine reference virus. Influenza B Yamagata viruses are all similar to the vaccine clade, and 70% of the Victoria viruses are similar to the vaccine reference viruses. Almost 800 viruses have been tested for resistance to oseltamivir, peramivir, and zanamivir. Of these, four H1 specimens have been found to have reduced susceptibility to oseltamivir and peramivir.

Outpatient visits for influenza-like illness (ILI) increased during the week to 4.3%. In the past five seasons, the peak percentage for ILI ranged from 3.6–7.5%. We have now been at or above the national baseline for 9 consecutive weeks. All 10 surveillance regions were above their region-specific baselines. On a state level, 24 states experienced high ILI levels, 10 states reported moderate activity, and the remaining 16 states were at low or minimal activity levels.

Cumulative reports to FluSurv-NET this season indicate a rate of 20/100,000 lab-confirmed influenza hospitalizations. Highest hospitalization rates were seen in persons >65 years at almost 53/100,000, followed by the 0–4 age group at 33.5/100,000. Ninety percent (90%) of the adults hospitalized for influenza reported at least one underlying medical condition (usually cardiovascular disease, obesity, or medical disorder). For children, at least 41% reported at least one underlying medical condition (usually asthma or obesity).

The percent of deaths reported due to pneumonia and influenza (P&I) has remained below the epidemic threshold for each week in this reporting season. The current week reports indicate 6.9% of deaths were due to P&I, which is just below the threshold of 7.0%.

Four new influenza-associated pediatric deaths were reported to CDC during the week, bringing the total number of deaths reported this season to 28. Fifty-four percent (54%) were associated with H1 infection, 7% with H3 infection, 26% with an influenza A virus infection which was not subtyped, and 4% with influenza B virus. Of the 26 children for whom a medical history was known, 54% did not have an underlying medical condition. Of the 22 children who were eligible for influenza vaccination, 6 had received at least one dose of vaccine, and three of the six were considered fully vaccinated.

For the geographic spread of influenza during the week, 47 states reported widespread activity, 2 states reported regional activity, and 1 state reporting local or sporadic activity.

For the preliminary influenza burden reports released, an estimated 13.2–15.2 million persons have been ill with flu. Approximately half of those persons have received medical care, and 155,000–186,000 were hospitalized. The MMWR released this week also included death information, with 9,600–15,000 deaths estimated to have occurred this season.

As noted in Table 2 of the publication, the overall estimate of vaccine effectiveness (VE) for all influenza A and B is 47% (confidence interval range of 34–57%). CDC’s message with this release is that the vaccine is working within the range of estimates we have seen in past years when the vaccine is well-matched with the viruses being seen. When examined by age group, the vaccine is most effective in children/adolescents age 6 months–17 years (61%). VE for persons 18–49 years was 37%. At 24%, the vaccine was least effective in persons >50 years. By type of influenza, the vaccine was 44% effective against influenza A (H3N2) and 46% effective against influenza A (H1N1). Specifically for H1N1, the vaccine was most effective in the 6 months–17 years age group (62%) and least effective in persons >50 years (8%). These estimates are slightly lower than what was reported in Australia in the 2017–2018 season and in Canada earlier this year. However, Josh emphasized that the U.S. reports are interim information based on small sample sizes. By the end of the year, increasing cases should provide better estimates, particularly in older age groups.

A question was asked about reports where the confidence intervals showed VE in negative numbers. Josh noted that we are seeing lower attack rates for adults in general, so a smaller number of influenza positive patients are available for inclusion in the development of these percentages. The good news is that the vaccine coverage rates in older adults are high, which contributes to the lower number of cases. L.J asked if this H1N1 season was consistent with the patterns seen in 2009, when older adults had lower attack rates. Josh agreed that this was the case.

Another Summit member noted that it is critical to continue to watch the mortality rate in older adults, even though the numbers seem to be low. He asked whether in the future we will be able to look at the differential effects of vaccine type (e.g., standard dose vs high dose) in older populations. Josh noted that CDC is looking forward to receiving VE estimates from patients who are hospitalized. Comparing VE estimates across patients for different levels of severity can prove quite helpful. At this time, CDC does not have sufficient information to be able to discuss VE by vaccine type. However, that should be available when the final VE estimates are issued at the end of the season.

Other Items – L.J Tan (IAC)

2019 Summit In-Person Meeting Information Now Available Online – The 2019 Summit in-person meeting will be held in Atlanta, GA on May 14–16, 2019. Information on registration, submission of poster abstracts, and nominations for the 2019 Immunization Excellence Awards is available on the 2019 National Adult and Influenza Immunization Summit webpage. (Please note that the password to register for this invitation-only meeting is available by contacting L.J Tan.) If possible, please book your room either through the onscreen “pop-up” that appears after you register or later through the email you receive after completing your registration. This not only will give you access to the “preferred rate,” it also will help the Summit meet its lodging contractual agreement with the hotel.

2019 Immunization Excellence Awards and Poster Submissions – Nominations are being accepted for these awards through February 15. Awards will be given in five categories. Nominations may be made online via the Summit website. The Summit also is accepting abstracts for poster presentations at the meeting.